dead spots

my moog etherwave plays beautifully, but has a weird antennae dead spot. when my hand is positioned about 1.5 inches from the left side of the pitch antennae, it makes absolutely no noise. this is annoying because the pitch that was there is entirely missing, and makes it (even more) difficult to play a song. does anyone know why this happens, or has this happened to anyone else? i don't even know if there's a term for this issue. it just started happening today after i got home from a gig. does anyone know how to fix this?

Hi everybody! I hope many people can help a complete novice like me. Here is my story.I borrowed a Moog Etherwave from a friend, kept it for five days. That is a very funny example of what I was able to achieve before I returned it: Smile. Next day I ordered my own Etherwave Plus Kit. Came without specific Plus instructions, took me some effort to get it from Moog Music. I built it. I have some problems with which I need help.When I first turned it on, the pitch response was reversed (it kind of tells me that the board was not tuned at factory as they claim.) I tuned it. And I think my tuning is bad.

Every little move of the Pitch knob from middle position throws the instrument in disarray: zero beat zone in the middle of the instrument space or close to pitch rod, too high etc. It plays pretty consistently at the middle position. However, the linearity is bad. And I am not complaining about squeezed space at highest pitches, it is easy to stretch it by moving at a slope. The low tones are too far apart.

It was very hard to achieve even that, because just putting the top on would change the tuning completely, not just a little bit. I had to go back multiple times to do a lot of guesswork.I noticed that the adjusting nut on the rightmost trimpot is very high up (far counterclockwise). If I try to tune it after advancing, the sound eventually becomes very soft, but not zero beat.

The amplifier produces audible humming and sometimes a second tone (it did not happen with the borrowed theremin)

The Waveform control does not seem to have any effect. And what is the purpose of shorting the C28 capacitor while tuning?

Sorry for the long post. I understand that this Techniques forum, but tuning is also a technique. Please help.

One should not build a theremin from a kit and try to tune the variable inductors as long as one has not enough understanding of what happens in the oscillators' circuits.

You should not have touched to the rightmost variable inductor (L5) since it is responsible for pitch range, linearity and much more. Correcting slightly the middle inductor (L6) would have done the job. When the pitch field seems to be reversed (zero point too close or even virtually "inside" the pitch antenna) means that the fixed pitch oscillator is on a too low frequency and has to be raised by turning L6 somewhat counterclockwise until the zero point is around 4" of the pitch antenna. This will give the correct field size when the cover is later closed.

Now, that L5 is de-tuned by simple stupidity and ignorance, you have to get a precision frequency counter in order to get quickly the initial tuning back (too much turning and experimenting with these ferrite cores will make them unusable). Turn the pitch knob to it's center position. Connect the frequency counter's probe to the base (middle pin) of transistor Q4 and adjust L6 to read 288300Hz. Then put your right hand at 4" or 10cm from the pitch antenna and adjust L5 until you get zero beat. When you close the cover now, you should be able to move the zero point by tuning the pitch knob from about 4"/10cm (fully clockwise) to more than 3'/1m from the pitch antenna (fully counterclockwise).

And that's it! Fix the cover with the screws and never touch again these coils!

I'm perhaps too old and have the "false" background, but I don't treat theremins as "simple" electronic circuits but with much more respect and "délicatesse", rather as a luthier would do with a violin. In the first place, a theremin is a music instrument!

You never would use a pneumatic drill in order to fix a problem of a violin or harp. That's why I always get into a paddy when I see that people start "adjusting" their theremin's oscillators without enough knowledge.

The purpose of shortening C28 during tuning is to disable the VCA and get an output signal at maximum volume without having to care about the volume antenna.

You have to understand that the pitch knob adds or subtracts several Hz from/to the fixed pitch oscillator. If it is i.e. turned to subtract 100Hz, this will even not change the audible tone by a half tone when playing at 2000Hz, relatively close to the pitch antenna. But the same turn will change a lower tone (one octave below middle C) by almost an octave.

That means that the lower you go, the more prudent your pitch knob must be turned in order to get optimal results. That means also that on an Etherwave you have almost no influence with the pitch knob on what happens in the highest range. The top octave will be compressed for technical reasons. The octave just below will have an almost invariable spacing, that's why I call it the "natural" octave spacing of a specific instrument. In order to get optimal linearity, I will now "feel" and memorize this distance with my right hand and then adjust the pitch knob finer and finer while "copying" this distance with my right hand octave-wise downwards, more and more away from the pitch antenna, always checking with the help of either absolute pitch or reference tones for exact octaves.

If the behavior of the knobs seems not to be ok, you may check with a digital multimeter at the 10-pin connector at the main circuit board:

black wire: -11.3V not depending on the knobs' positiongreen wire: 0V (Ground) not depending on the knobs' positionblue: Pitch tuning, varies between -11.3V and 0V depending on the pitch knob's positionviolet: Volume tuning, varies between -11.3V and 0V depending on the volume knob's positionred: Waveform, varies between -11.3V and 0V depending on the waveform knob's positionorange: +12V not depending on the knobs' positionbrown: Brightness, varies between 0V and +12V depending on the brightness knob's position

Thierry, I appreciate very much this detailed description. I am sure that the Pitch control behaves as it should, given the incorrect setting of L5, or at least empirically it could not be evaluated because of the incorrect tuning. It works, it just does not add or shift the pitch, it abruptly changes it.

It was the Waveform control that did not seem to do anything at all. Thanks for the wire breakdown. I will check if my solderwork is to blame.

Awesome stuff, exactly what I have been hounding you for. I will try these adjustments on my EtherWave Standard. Linearity improvements on an EWS only has a few adjustments besides the magical wire?

Now for your amusement, I bought several vacuum tube/valves with the same ID number but from different manufacturers. I found them to be like a box of chocolates:

For the theremin voice [i]"you just never know what you're going to get!"[/i]

Thanks Thierry, one day I will visit a European theremin event, nothing holds me back except 8 hrs jet lag and have never been on a jet! LOL

PS: The gentleman that started this thread a while back is in So. California I believe, now if I could get his magical skill to record a demonstration of my theremin instrument that would be interesting? (-: